1. Field of the Invention
The present invention principally relates to the electrical and/or thermal interconnection of electronic components, such as integrated circuit devices, which have a large array of closely spaced conductive contact sites. More particularly, the present invention is an anisotropically conductive matrix of a dielectric material with at least one electrically and/or thermally conductive element extending therethrough to provide electrical and/or thermal conduction in at least one direction through the matrix. Even more particularly, the present invention is an elastomeric matrix having at least one via containing a conductive member made up of electrically and/or thermally conductive elements, preferably in a polymeric binder. The conductive elements have a maximum dimension of: (i) about 5% to about 120% of the length of the vias through the matrix, and, (ii) about 10% to about 100% of the width of the via. The size and shape of the conductive elements, as well as the properties of the binder and the matrix material, may be selected to provide a conductive member in each via with specified electrical and/or thermal properties and rigidity to form a reliable interconnection between the contact sites on the electronic devices.
2. Description of Related Art
To alleviate many of the problems associated with C-4 or solder ball connection systems while achieving a high density of electrical connections, it is desirable to have a flexible connection between an electronic device and the package to which the device is electrically interconnected and mechanically mounted. Many examples are known in the art of anisotropically conductive interposers for electrical connection which consist of a resilient dielectric matrix material having a discrete configuration of conductive elements extending transversely therethrough. While the properties of the dielectric matrix material play a significant role in electrical interconnection, it is also understood that the properties of the conductive elements also have a significant effect on the quality of the connection provided by the resilient interposer.
For example, the conductive members in the connector must be sufficiently resilient to accommodate differences in height among contacts on the electronic devices to be interconnected, to prevent undue stress on the fragile contacts, and to provide a reusable structure for testing and burn-in applications. However, the conductive members in the resilient connection system must be sufficiently rigid to provide contact wipe to penetrate surface films, debris and oxide layers on the mating contacts on the devices to minimize contact resistance.
For example, U.S. Pat. No. 5,049,085 to Reylek describes an anisotropically conductive polymeric matrix including a polymeric layer or film, preferably adhesive in nature, with a plurality of conducting members therethrough. The conductive members, which are preferably tubular in design, are preferably made of metals deposited on the walls of the orifices in the polymeric layer at a thickness of about 100 .ANG. to about 0.005 inch (0.013 cm). The '085 patent states that as the thickness of the conductive surfaces increases, the conductor wall becomes stiffer and less deformable, which decreases contact efficiency by failing to accommodate variations in height and shape of the mating contacts on the electronic devices to be interconnected.
To accommodate such height/shape variations in the mating contacts, it is well known in the art that the conductive members in the resilient matrix may comprise a conductive powder in a resilient binder. For example, U.S. Pat. No. 4,008,300 to Ponn describes a multi-conductor element for interconnecting circuit boards and the like. Conductive material consisting of a slurry of a conductive powder in a resilient liquid vehicle is filled into the gaps created in an elastomeric matrix, with the conductive material protruding above the surface of the elastomeric matrix. The pressure engaged connections provided by the resilient contacts in the connector described in the '300 patent protect delicate electronic devices from damage during connection procedures and accommodate variations in the shape and structure of mating contacts, but the combination of a conductive powder in an elastomeric vehicle does not provide a conductive member that is sufficiently rigid to provide the preferred wiping action on the surfaces on the mating contacts. A similar lack of wiping action might be expected from the connector described in U.S. Pat. No. 5,037,312 to Casciotti et al., in which the conductive members in the elastomeric matrix material consist of a resilient gel loaded with conductive particles.
U.S. Pat. No. 5,275,856 to Calhoun describes an electrically conductive adhesive tape with at least one flexible carrier web that has a low adhesion face bearing on an adhesive layer. The tape is formed with a plurality of perforations, each of which is filled with electrically conductive particles. The conductive particles may include metals, metal coated polymeric particles and graphite. The particles used in the examples of the '856 patent had a Fisher subsieve size of about 2.2 to about 7 .mu.m, and the particles were placed in vias in the adhesive matrix having a maximum breadth of about 0.1 mm and a depth of at least 0.5 mm. The conductive adhesive tape was capable of carrying about 10 amps/cm.sup.2 through its thickness.
In preferred applications, the perforations in the Calhoun '856 patent also contain not only the conductive particles, but an organic binder, which can bind the particles in each perforation into durable, column-like conductive members. The binder may be selected to influence the properties of the conductive member, e.g., to permit collapse of the columns when a connection is made, to provide resilience, or to add to the adhesive bond provided by the adhesive layer. When used, the binder typically contributes from about 1 to about 50% of the volume of the conductive member, although the volume fractions of conductive particulate may be adjusted to control electrical conductivity.
The '856 patent states that when the conductive particles in the vias are soft, moderate hand pressure applied to the mating contacts on the electronic devices to be interconnected can flatten the particles to provide a small, flat conductive area where each particle contacts another particle or an electrode. Thus, as with the other conventional connector systems with conductive particles discussed above, the conductive members in the '856 protect delicate electronic devices from damage during connection procedures and accommodate variations in the shape and structure of mating contacts. However, the combination of a conductive powder in an elastomeric vehicle distributes contact pressure uniformly, and is not sufficiently rigid to provide high local pressures at the contact surfaces to create a wiping action on the surfaces. In addition, the multiplicity of contact points among the small particles in the vias would be expected to create an excessively high resistance to current flow.
Thus, a need exists in the art for a conductive member having the resilience to provide reusability and to accommodate variations in the shape and structure of the mating contacts on electronic devices, but reserving some measure of rigidity to provide high local pressure and a wiping action at the surface of the mating contacts. The conductive member must also conduct significant amounts of current and/or thermal energy between mating contacts, and must have a low resistance to the flow of electrical and/or thermal energy.